1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers, and more particularly, to a dielectric filter and a dielectric duplexer for use in a communication unit in the microwave band or the millimeter-wave band.
2. Related Art of the Invention
To provide a dielectric filter which does not have a large transmission loss even in a frequency band of 3 GHz or higher, the dielectric filter shown in FIG. 22 was proposed in Japanese laid-open patent publication No. 4-287502. In the dielectric filter 80 shown in FIG. 22, three TE.sub.10 -mode resonators 81a, 81b, and 81c are connected in series.
As shown in FIG. 23, two resonators 81a and 81b disposed at both ends of the dielectric filer 80 are provided with outer conductors 83a and 83b formed on the surfaces of dielectric blocks 87a and 87b so as to substantially cover the dielectric blocks 87a and 87b. At ends of the resonators 81a and 81b, input and output electrodes 82a and82b are formed. On respective connection planes at corresponding ends of the resonators 81a and 81b, the outer conductors 83a and 83b extend, except for circular coupling windows 84a and 84b.
The resonator 81c disposed at the center of the dielectric filter 80 is provided with an outer conductor 83c formed on the surface of a dielectric block 87c so as to substantially cover the dielectric block 87c. On connection planes at both ends of the resonator 81c, the outer conductor 83c extends, except for circular coupling windows 84c and 84d. These three resonators 81a to 81c are connected in series by bonding the outer conductor 83c extending to the connection planes at both sides of the resonator 81c to the outer conductors 83a and 83b extending to the connection planes of the resonators 81a and 81b with electrically conductive adhesive such as solder to form the dielectric filter 80.
L-shaped metallic lead electrodes 86a and 86b are mounted by soldering to the input and output electrodes 82a and 82b, respectively, provided at both ends of the dielectric filter 80. The dielectric filter 80 is connected to an external circuit via these lead electrodes 86a and 86b.
A dielectric filter 90 shown in FIG. 24 has also been proposed. The dielectric filter 90 has substantially the same structure as the dielectric filter 80 except that resonators 81a, 81b, and 81c are connected in series by the use of fixing metal fittings 85a and 85b. In other words, as shown in FIG. 25, gaps are provided between the resonators 81a and 81c and between the resonators 81c and 81b to allow the relative positional relationships among the resonators 81a to 81c to be easily changed. The positions of the resonators 81a to 81c are adjusted as required in order to obtain the desired electromagnetic coupling among the resonators 81a to 81c. Mutual electromagnetic coupling among the resonators 81a, 81b, and 81c can be easily adjusted in this way.
After the relative positions of the resonators 81a to 81c are adjusted, the resonators 81a and 81c are supported and connected by a fixing metal fitting 85a and the resonators 81c and 81b are supported and connected by a fixing metal fitting 85b to form the dielectric filter 90. L-shaped metallic lead electrodes 86a and 86b are mounted by soldering to the input and output electrodes 82a and 82b, respectively, provided at both ends of the dielectric filter 90.
However, since the dielectric filters 80 and 90 are connected to an external circuit via their input and output electrodes 82a and 82b and via the L-shaped metallic lead electrodes 86a and 86b, a low degree of freedom is provided in terms of the lead direction and the lead position due to restrictions on the material and shape of the lead electrodes 86a and 86b. Therefore, the degree of freedom in designing the patterns of the external circuit to be connected to these lead electrodes 86a and 86b is also low.
In addition, since the dielectric filter 80 has a structure in which the resonators 81a to 81c are bonded by electrically conductive adhesive applied to the connection planes, mutual connection strength among the resonators 81a to 81c is relatively low. Therefore, if bending stress is applied to the dielectric filter 80, a crack or peeling off may occur at the bonded sections of the resonators 81a to 81c.